1. Field of the Invention
The present invention relates to a ceramic substrate for mounting electronic components and the like thereon, and more particularly to a surface structure of a ceramic substrate having superior heat resistance and method of manufacturing the same.
2. Description of the Background Art
A surface of a ceramic substrate is metallized to mount or seal electronic components. With reference to FIG. 1 showing a conventional surface structure of a ceramic substrate, a refractory metal 2 such as W or Mo is formed by baking on a ceramic substrate 1 of alumina, for example, an Ni plating layer 3 is formed thereon, and an Au plating layer 4 is formed further thereon. The Ni plating layer 3 facilitates soldering of the electronic components or the like, and serves to protect the metallized layer 2 formed thereunder. The Au plating layer 4 prevents oxidation of the Ni plating layer 3 formed thereunder to provide superior connection between the Ni plating layer 3 and the electronic components, thereby maintaining superior reliability of the electronic components and the like.
When elements are to be bonded on the Au plating by soldering, the ceramic substrate or the surface of the ceramic substrate is heated. However, the uppermost Au layer is tarnished by the heating, which prevents superior bonding. The cause of the tarnish of the Au plating layer may be diffusion of Ni of the Ni plating layer into the Au plating layer caused by heating, in which Ni is coupled with oxygen introduced from the air, providing Ni oxide.
In order to avoid the above described problem, the thickness of the Au plating layer is generally made as thick as about 2 .mu.m. However, in order to reduce cost, the Au plating layer should preferably be reduced in thickness.
Japanese Patent Laying Open No. 59-114846 discloses a method of suppressing diffusion of Ni in the Ni plating layer to the Au plating layer. More specifically, after the Ni plating layer is formed, the Ni plating layer is heat treated with a temperature range of 900.degree. C. to 1400.degree. C. in a reducing atmosphere, so as to improve heat resistance of the Ni plating layer.
However, the above method cannot sufficiently prevent tarnish of the Au plating layer. Especially when the thickness of the Au plating layer is not larger than 1 .mu.m, the Au plating layer is tarnished when electronic components and the like are bonded.